Referring now to the accompanying drawings, description is provided of a conventionally available spindle motor and a disk drive unit of the prior art.
FIG. 8 is a sectional side view showing a general structure of the disk drive unit equipped with a conventional spindle motor. In this figure, the disk drive unit is sectioned along a plane including a rotor shaft to illustrate the main structure of the disk drive unit equipped with the spindle motor.
In FIG. 8, rotor hub 82 is fixed to rotor shaft 81 by such means as press fitting and the like. Rotor hub 82 constitutes rotor unit 85 together with rotor magnet 83 and annular stopper ring 84, both of which are mounted to rotor hub 82. Stator-side bearing member 89, securely fixed to chassis 86, comprises bearing sleeve 87 having protruded flange 87a of a collar like shape and thrust plate 88 fixed to bearing sleeve 87. Rotor shaft 81 is formed in its surface with a dynamic pressure generating groove in a shape of herringbone pattern or the like, and it is inserted into a bore in the stator-side bearing member 89 with a very small clearance. Rotor shaft 81 and stator-side bearing member 89 compose a fluid bearing which functions as a radial bearing, and stator-side bearing member 89 rotatably supports the rotor shaft 81 in the radial direction. In addition, the thrust plate 88 constituting the stator-side bearing member 89 also has a dynamic pressure generating groove of herringbone pattern or the like formed in it. Thrust plate 88 thus composes a thrust bearing to support the rotor shaft 81 in a freely rotatable manner in the axial direction by a dynamic pressure generated between the bottom end of the rotor shaft 81 and the thrust plate 88 during rotation of the rotor shaft 81. A space formed of the small clearance between rotor shaft 81 and the stator-side bearing member 89 composed of bearing sleeve 87 and thrust plate 88 is filled with hydrodynamic lubricant 90. Furthermore, the chassis 86 is provided with stator 91 comprising coil 91b wound around stator core 91a, to complete the spindle motor 92.
Disk 93 having a recording layer (also called a recording film instead of recording layer) formed on the surface thereof is secured to a flange portion of rotor hub 82. Also provided are a signal conversion element (not shown in the figure) and swing means (not shown) for positioning the signal conversion element, as are well known, and all of the above components constitute the disk drive unit for recording and reproducing signals in and from the recording layer on disk 93.
Stopper ring 84 attached to rotor unit 85 composes a means for positively preventing the rotor unit 85 from slipping out of the stator-side bearing member 89, since the stopper ring 84 attached to the rotor unit 85 comes into sliding contact with the protruded flange 87a of the bearing sleeve 87, even if the disk drive unit receives an excessive impact and the like (refer to Japanese Patent Laid-open Publications, Nos. H08-275447, H11-055900, H06-233495 and H09-247886, for example).
The conventional spindle motor 92 of the above structure, however, has a problem that the assembling process is very complex due to a dimensional relation in which an inner diameter of the annular stopper ring 84 is smaller than an outer diameter of the protruded flange 87a of the bearing sleeve 87. In other words, the rotor shaft 81 is inserted first into the bore in the stator-side bearing member 89 composed of bearing sleeve 87 and thrust plate 88, after the rotor magnet 83 is secured to rotor hub 82 carrying the rotor shaft 81 attached thereto, and, only then the annular stopper ring 84 is fixed to the rotor hub 82 by any of the well-known methods such as bonding with adhesive, press-fitting, caulking, laser welding and the like, while the stator-side bearing member 89 carries hydrodynamic lubricant 90 filled in the space in which the rotor shaft 81 is inserted. During these processes, certain problems occur such that refuse of adhesive used for bonding, metal dust produced and spattered by press-fitting, caulking, or welding may be left attached to or caught between bearing sleeve 87 and rotor unit 85. Another problem may also occur in the reliability of adhesive when used for bonding, that hydrodynamic lubricant 90 remaining on surfaces of the rotor hub 82 reduces bonding strength of the adhesive. Furthermore, a pressure is transferred to the bearing sleeve 87 through the thrust plate 88 because a depressing force is impressed upon the upper end of rotor shaft 81 in the axial direction, if press-fitting method is used to fix the stator-side bearing member 89 assembled with the rotor unit 85 to chassis 86. This causes a damage of compression and the like to at least one of thrust plate 88 and rotor shaft 81 where they come into abutment, or loosens the thrust plate 88 fixed to the bearing sleeve 87, which is likely to lead into leakage of the hydrodynamic lubricant 90 contained in the radial bearing and the thrust bearing serving as fluid bearing. Moreover, if adhesive is used for fixing the stator-side bearing member 89 to the chassis 86, there is a possibility that refuse of the adhesive remains in the spindle motor, for the same reason as in the case of stopper ring 84, which gives rise of another problem of failing to ensure the reliability.